Non-volatile memories (NVMs) are generally characterized by their ability to retain stored data when power is turned off or when power is temporarily interrupted. Under similar conditions, other memory technologies, such as synchronous RAM (SRAM) and dynamic RAM (DRAM), for example, lose the stored information. One type of NVMs consists of read-only memories (ROMs), also referred to as masked ROMs. Data is stored into a ROM during production, for example, and may not be altered by a user.
Another type of NVMs consists of programmable ROMs (PROMs). In PROMs, a user may store or program data into the device by, for example, connecting links in the device to provide interconnections to the memory storage elements. Some types of PROM devices may only allow a user to program data into the device once. Other PROM technologies, such as erasable programmable ROMs (EPROMs), electrically erasable programmable ROMs (EEPROMs), and flash memories, for example, may support multi-stage programming that enables the user to program the device multiple times. For example, ultraviolet light may be utilized to erase data stored in EPROMs. The EPROM memory cell may consist of a single transistor, enabling memory devices with high storage densities. However, the use of EPROMs may be limited because removal from the system where the device is being used is generally required for erasure. Moreover, selective erasure of memory locations may be quite difficult to achieve, using for example, ultraviolet light erasure methods.
EEPROM technology, for example, supports electrical programming of data and electrical erasure of stored data. In this regard, EEPROMs utilize a WRITE operation for programming data into selective memory locations and an ERASE operation to erase data from selective memory locations. The EEPROM cell utilizes two transistors and a tunnel oxide and is generally larger in size than an EPROM memory cell. EEPROMs are generally specified based on the number of write-erase cycles that may be performed before failure.
Flash technology, for example, utilizes a single transistor memory cell that provides hot carrier programming and tunnel erase operations. In this regard, flash technology combines the programmability of the EPROM and the erasability of the EEPROM. The term “flash” refers to the ability of erasing the entire memory device or a large portion of the memory device using a single erase operation. The size of the flash memory cell make flash memory devices cost competitive with DRAMs.
A NVM device, whether a ROM device or a PROM device, may comprise an array of NVM blocks, where each NVM block comprises rows and/or columns of memory cells. In some instances, the production process may result in a large number of defects in the NVM device that may render the device unusable. These defects may be detected during production testing or during programming operations, for example. The defects may include damaged or non-operational memory cells, bit and/or word lines, write drivers, and/or sense amplifiers, for example. The cost of producing NVMs increases as a result of low production yields from high numbers of defects.
In order to increase production yields, and therefore compensate for the presence of existing defects, redundant elements may be utilized in the design of NVMs to replace defective elements. However, the use of redundant elements alone may not be an effective solution in those instances when the number of defects is very high. Since many existing applications and/or systems utilize NVMs, there is a growing demand for NVM architectures that are more robust to defects that may result from the production process.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.